Oxalic acid is, of course, a chemical substance. At high concentrations, it is a dangerous poison, but such immediately toxic levels are not found in foodstuffs but rather in
manufactures, such as some bleaches, some anti-rust products, and some metal cleaners (among other things). It is also a naturally occurring component of plants, and is found in
relatively high levels in dark-green leafy foods (relatively high, though, is just that).

The chemical formula for oxalic acid is C2O2(OH)2. An acid (from the Latin acidus, meaning "sour") is typically a corrosive substance with a sharp, sour taste (but tasting an acid can be
extremely dangerous, depending on its strength). Acids can range from very mild to very strong, and a given type of acid can be made weaker by diluting it (with, for example, water). Oxalic acid is inherently a strong acid: it is about 3,000 times stronger than acetic acid, which is the chemical name for the acid in ordinary vinegar (usually sold as around a 5% solution of acetic acid). Oxalic acid is so strong that it is widely used industrially for bleaching and heavy-duty cleaning, notably for rust removal. If oxalic acid is not heavily diluted—as it is in plants--it is quite dangerous to humans, being both toxic and corrosive.

The effects of oxalic acid in the human body, when ingested in foods, flow from its ability to combine chemically with certain metals commonly found in--and important to--the human body, such as magnesium and calcium. When oxalic acid combines with such metals, the result is, in chemical terms, a "salt" (table salt is just one specimen of the general class of salts); those oxalic-acid+metal salts are called oxalates. Since oxalic acid is not (so far as is known today) a useful nutrient, it is--like all such unneeded components of diet--processed by the body to a convenient form, those oxalates, and that byproduct is then eventually excreted--in this case, in the urine.

The potential problems with oxalates in the human body are two. First, they mean that the metal in them--say calcium--has been made unavailable to the body; if a large amount of oxalic acid is ingested, the oxalates formed mean that the body is being to some degree deprived of certain essential nutrients. For normal, healthy persons, that risk is nearly trivial provided that great amounts of oxalic acid are not consumed on a continuing, long-term basis. The second effect is not chemical but mechanical: the crystals of oxalate, very small but very sharp, can be large enough to irritate the body. The chiefest and most famous example of this is kidney stones--probably 80% of kidney stones derive from calcium oxalate.

The extent to which foods high in oxalic acid are a potential health problem varies from person to person. Individuals with especial vulnerability to oxalates--notably those with kidney disorders, gout, rheumatoid arthritis, or certain forms of chronic vulvar pain (vulvodynia)--need to be careful about their intake of oxalic acid. Normally healthy people probably do not, unless, as noted before, they are consuming unusually large amounts of oxalic acid on a long-term continuing basis.

Despite the plethora of articles on the web, there is little hard data--many references are either from inherently unreliable sources, or seem to be parroting material they scarcely
understand. Here is what we have gleaned.

Sheer toxicity--actual poisoning--from ingested oxalic acid is wildly unlikely. The only foodstuff that contains oxalic acid at concentrations high enough to be an actual toxicity risk
is the leaves--not the stalks, which is what one normally eats--of the rhubarb plant. (And you'd need to eat an estimated eleven pounds of rhubarb leaves at one sitting for a lethal dose, though you'd be pretty sick with rather less.) For just about every other foodstuff, the risk--if any--is not immediate toxicity but a contribution to the development of oxalate crystals.

Some have argued that by readily combining with calcium, oxalic acid in the diet reduces one's effective intake of dietary calcium. That is true, but the size of the effect is, for anyone getting decent nourishment, not meaningful. Even the conservative RDA for calcium is a gram or so (1000 mg) a day, and many believe that 1.5 to 2 g a day is better. As one source put it: "While research studies confirm the ability of phytic acid and oxalic acid in foods to lower availability of calcium, the decrease in available calcium is relatively small." And the NIH (National Institues of Health) states that "For people who eat a variety of foods, these interactions probably have little or no nutritional consequence and, furthermore, are accounted for in the overall calcium DRIs [Dietary Reference Intakes], which take absorption into account."

Nor need one be afraid to boost one's calcium intake. The belief that high calcium intake aggravates the formation of kidney or bladder stones has now been pretty well discarded, with
studies showing that even intakes well above 2 g/day do not participate in stone formation in persons who do not otherwise have a stone problem. In fact, some studies suggest that calcium-loading (as by drinking milk) when ingesting foods high in oxalic acid helps the body to better absorb and dispose of the oxalic acid. Further, getting decent amounts of potassium in one's diet will also minimize the effects of calcium participation in stone formation for those who do have a problem. Worth noting in this connection is that magnesium improves the absorption of ingested calcium, so making sure to maintain a proper dietary balance of the two (often given as 2:1 calcium:magnesium) is also important.

It is now generally believed that the normal human body can dispose of oxalic acid at even relatively high dietary quantities without trouble; though it is very poorly absorbed (having no metabolic use), it is readily enough excreted. Trouble comes only to those unfortunate enough to have one or another condition--usually genetic in origin--that impairs, to a greater or lesser degree, their bodies' ability to process oxalic acid. (Though sometimes stones and gout are not related to oxalic acid at all.) For those folk, oxalic acid is not the cause of their problems, but it is the raw material for it, and they do indeed need to regulate their intake of it, just as diabetics need to monitor their sugar intake despite sugar normally being a harmless substance.

Those with a need for caution include sufferers from kidney disorders and kidney stones, gout, rheumatoid arthritis, and certain forms of chronic vulvar pain (vulvodynia).

(Gout is a painful condition caused by crystals, such as oxalates, being deposited in the extremities, typically the feet; it is thus clearly related to kidney- and bladder-stone conditions.)

Unfortunately, a simplistic tabling of "oxalic-acid content" is not enough to compare foods, for at least four reasons:

The kind of oxalic acid: as one study reported, "The higher oxalate absorption from [food #1] than from [food #2] suggests that the relative amount of soluble and insoluble oxalate in food has an important role in the determination of oxalate absorption [emphasis added]. Simple percent-composition lists make no such distinctions.

The actual food on your dish: it has been shown that the
oxalic-acid content of foodstuffs can vary substantially depending on their growth environment
(for example, plants with lots of ammonia available to them when growing had substantially
lower oxalic-acid contents).

Again the actual food on your dish: how was it prepared? One tabulation shows "boiled
spinach" at 0.60% oxalic acid but "frozen spinach" at 0.75% oxalic acid, a 25% increase.

Different numbers from different sources: perhaps the differences only reflect the some
of factors cited above, but they are real and sometimes substantial. There is a table from
a 1984 USDA publication, which is the ultimate source of most oxalic-acid tabulations to be
found on the web; but there are other, newer, and apparently quite trustworthy tabulations (see
the list below) that often disagree with the USDA-table data, and with each other as well. Such
a situation is, to use a technical term of the medical profession, insane.

So any simplistic, straightforward tabulation of the "oxalic-acid content" of foodstuffs is thus a tricky thing at best. One has to look at such listings with an understanding that
they are likely to be only broad-brush indicators--what is relatively high, what is relatively low--not exact, reliable numbers.

For lack of a better idea, we present here, side by side, oxalic-acid data from three
sources:

Phytochemical and Ethnobotanical Databases, Agricultural
Research Service of the National Genetic Resources Program (most data taken from Dr. James
Duke's 1992 manual, Handbook of phytochemical constituents of GRAS herbs and other economic
plants, Boca Raton, FL; CRC Press)--at that page, enter oxalic acid and click
Submit Query

LithoLink Corporation, a metabolic testing and disease
management service for kidney stone patients (founded by Dr. Fredric Coe, a University of
Chicago Medical School Professor)

Both Duke and Litholink give concentrations to much more than two decimal places, but we have rounded off their data to match the low precision offered by the simplistic USDA, to make
comparison simpler. As you can see, sometimes they are all in good agreement, as with corn; sometimes they are not together but not wildly apart, as with beans; and sometimes they are on
different planets, as with carrots. What the consumer is supposed to make of such an insane mish-mash is beyond our ken.

We have fallen back to using light and dark red to highlight those foodstuffs that seem worth paying attention to if one has a susceptability to oxalic acid, but that is only a rough
guide. Those in that condition should get medical advice and follow it.